Revert to the original logo.
Add spacing around CMI button, replace ackno --> Credits.
libobjcryst is external library.
Shorten license reference so it fits on one line.
On MacPorts use gsl instead of gsl-devel.

gsl-devel is not maintained, gsl package is.
Reformat and expand on MacPorts.

Add diffpy-dev link for installation help queries.